The proposed studies are unified by their common aim of elucidating general principles of neuromodulation and plasticity in experimentally tractable model systems found in lower vertebrates and invertebrates. Three individual projects which share common approaches and methodologies are included: (I) Role of mGluRs in Oscillatory Neuronal Circuits, (II) Specificity and Plasticity of Developing Synapses, (III) Modulatory Neuropeptides and the Integration of Behavior. All of these studies will provide insight into the basic processes underlying such phenomena in more complex nervous systems. So called simple systems have traditionally been used to help explain such fundamental mechanisms as the ionic basic of the resting and action potential, the quantal nature of synaptic transmission and the circuitry underlying rhythmic motor pattern generation. Such basic research will eventually lead to a rational basis for the diagnosis and treatment of a variety of neurological diseases. Four separate core facilities are requested: Administrative; Histology, Animal and Computer. The individual projects will all use the general core facilities extensively as they have in the past. These studies constitute one of the major research efforts at the Institute of Neurobiology and of the Medical Sciences Campus of the University of Puerto Rico. The Institute is an interdisciplinary, interdepartmental research facility devoted to the study of the structure and function of the nervous system with an emphasis on comparative studies at the cellular and systems level. If offers investigators and students from Puerto Rico, the rest of the United States and the Americas, as well as Europe, access to the rich Caribbean marine, terrestrial and fresh water fauna for neurobiological investigations. The Institute of Neurobiology serves as a focus for bringing together researchers who are interested in various aspects of contemporary neuroscience. The number of active researchers in this field in Puerto Rico has been quite small but is steadily growing by building on the foundation provided by the Program Project Grant. The number of collaborations between Institute scientists and scientists from all over the Island is steadily growing and this Grant provides an important catalyst for a continuation of this growth.